Assessment (as of 30 November 2017):
- In the past month or so, sea surface temperatures of the central and eastern equatorial Pacific continued to cool. Based on the latest oceanic observations as well as forecasts by a number of climate models around the world, it is expected that La Niña may develop in winter 2017/18 (December 2017 – February 2018). Statistical analyses indicate that when La Niña occurs during wintertime, winter temperatures in Hong Kong tend to be generally lower than those during the ENSO-neutral state. On the other hand, under the influence of global warming and local urbanization, winter temperatures in Hong Kong reveal a significant long-term rising trend, partially counteracting the effect of La Niña. Taking all available information into consideration, including prediction from climate models around the world as well as forecast guidance based on atmospheric and sea surface temperature factors, it is forecast that the overall temperature in winter 2017/18 in Hong Kong will tend to be normal to above-normal.
- Although the overall winter temperature in Hong Kong is expected to be normal to above-normal, fluctuations in the temperature could still be quite large. Under the influence of La Niña, the chance of occurrence of cold weather for Hong Kong this winter would be higher than that of last year. Please refer to the latest assessment provided by the local weather forecast and the 9-day weather forecast issued by the Hong Kong Observatory. (Cold weather refers to the temperature falling to 12 °C or below with 17 cold days on average each year. During last winter, there were a total of 7 days of cold weather. )
- The impact of La Niña on winter rainfall in Hong Kong is not significant. The majority of climate models predict that the rainfall over southern China is likely to be normal to below-normal in winter 2017/18.
- Climate prediction centres around the world generate seasonal forecasts, employing a variety of methods including
dynamical models, statistical methods, expert judgment and combinations of them. Predictions from different centres do
not always agree and large discrepancies can occur at times. The Observatory adopts an ensemble approach to formulate
its seasonal forecast for Hong Kong, taking into consideration available products from major climate prediction centres
and the Global-Regional Climate Model (G-RCM) operated in house. Thus,
the resulting forecast issued by the Observatory may be different from the forecast given by G-RCM.
- The Observatory gratefully acknowledges the Tokyo Climate Center of the Japan Meteorological Agency and the National
Centers for Environmental Prediction of National Oceanic and Atmospheric Administration, USA for providing dynamical
prediction model forecast and hindcast data to support the formulation of seasonal forecast for Hong Kong.
- Forecast for spring 2018 (March to May 2018) will be available around 1st March 2018.
Seasonal forecast charts provided by G-RCM and major climate centres: